1. Field of the Invention
The invention relates to a technology of a continuous gas carburizing furnace capable of arbitrary selection between gas quenching and oil quenching.
2. Description of the Related Art
A known method of surface hardening performed on a steal material (hereinafter, termed “workpiece”) according to the related art is a carburizing process. The carburizing process is a method in which a surface of a workpiece is infiltrated with carbon (carburized) and the carbon in the surface is diffused so as to increase the amount of carbon in the surface, and then quenching is performed so as to improve the abrasion resistance of the workpiece's surface while securing toughness of the workpiece.
Among the carburizing processes, a gas carburizing method that uses a carburizing gas (CO gas) as a carburizing agent is known. In fact, a carburizing process that uses a continuous gas carburizing furnace is often employed because, among other reasons, this method is able to carburize a large quantity of workpieces at a time.
With reference to FIG. 10, an example of the continuous gas carburizing furnaces according to the related art will be described. FIG. 10 is a side sectional view showing an overall construction of a continuous gas carburizing furnace 101. For the following description, it is to be noted that the direction of an arrow A in FIG. 10 shows the conveying direction of workpieces 50, and defines the forward direction of the continuous gas carburizing furnace 101.
The continuous gas carburizing furnace 101 is made up mainly of a degreasing chamber 102, a preheating chamber 103, a carburizing chamber 104, a diffusion chamber 105, a temperature decrease chamber 106, an oil quenching chamber 107, etc. These chambers 102, 103, . . . , 107 are contiguously arranged in a line along the conveying direction of the workpiece 50 (the direction of the arrow A in FIG. 10). Then, the gas carburizing process is performed on the workpiece 50 by the following series of operation processes: (1) a grease or the like adhering to a surface of a workpiece 50 is removed therefrom in the degreasing chamber 102; (2) the temperature of the workpiece 50 is increased in the preheating chamber 103 to a temperature suitable for the gas carburizing process; (3) a carburizing gas (CO gas) is blown to the surface of the workpiece 50 in the carburizing chamber 104, so that carbon is infiltrated into the workpiece 50 from its surface; (4) the workpiece 50 is kept at a predetermined temperature in the diffusion chamber 105, so that the carbon (atoms) infiltrated in the workpiece 50 diffuses; (5) the temperature of the workpiece 50 is decreased in the temperature decrease chamber 106 to a temperature suitable to quenching; and (6) the workpiece 50 is placed into the oil quenching chamber 107, so that the quenching process is performed on the workpiece 50.
In the foregoing continuous gas carburizing furnace 101, the workpiece 50 is continuously conveyed by a conveying device made up of a roller conveyor or the like which is disposed inside the furnace, so that the gas carburizing process is performed as the workpiece 50 passes through the chambers 102, 103, . . . , 107 in that order. Therefore, it becomes possible to continuously process a plurality of workpieces 50, and thus high productivity can be achieved.
Incidentally, as for the quenching process performed after the surface of the workpiece is infiltrated with carbon (carburized) and the carbon in the surface is diffused, gas quenching as well as the foregoing oil quenching is known, and the two quenching processes have different characteristics. That is, in the oil quenching, many workpieces are submerged directly into an oil tank at a time, so that productivity is high. However, since the workpieces are rapidly cooled in a short time, local distortion is likely to occur, and high precision quality (product accuracy) is difficult to secure. On the other hand, in the gas quenching, workpieces are cooled by a gas, that is, an inert gas (nitrogen gas), so that a longer cooling time is required than in the oil quenching, and therefore lower productivity results. However, since workpieces are cooled gradually as a whole, local distortion is unlikely to occur, and high precision quality (product accuracy) can be secured.
Comparison in the product accuracy of workpieces between the oil quenching and the gas quenching will be described with reference to FIGS. 11A and 11B. FIGS. 11A and 11B are bar charts showing comparison between the oil quenching and the gas quenching in terms of the product accuracy of gears (toothed wheels) as an example of workpieces. The chart of FIG. 11A shows the shape accuracy, and the chart of FIG. 11B shows the tooth surface accuracy. Incidentally, the “shape accuracy” refers to the post-quench amount of eccentricity of the external shape of an entire gear relative to the pre-quench amount thereof. Besides, the “tooth surface accuracy” refers to the post-quench amount of distortion of the shape of each gear tooth surface relative to the pre-quench amount thereof.
In FIG. 11A, the vertical axis shows the “shape accuracy”, and higher values in the “shape accuracy” mean larger amounts of eccentricity of the entire external shape of the gear. That is, on the vertical axis, higher values in the “shape accuracy” indicate lower degrees of the shape accuracy, and lower values of the “shape accuracy” indicate higher degrees of the shape accuracy. Therefore, by comparison in the shape accuracy between the oil quenching and the gas quenching in the bar chart presented in the foregoing fashion, it is apparent that the bar of the gas quenching is smaller in value than the bar of the oil quenching, showing that the gas quenching is higher in the shape accuracy than the oil quenching.
In FIG. 11B, the vertical axis shows the “tooth surface accuracy”, and higher values in the “tooth surface accuracy” mean larger amounts of distortion of the shape of each tooth surface of the gear. That is, on the vertical axis, higher values in the “tooth surface accuracy” indicate lower degrees of the tooth surface accuracy, and lower values in the “tooth surface accuracy” indicate higher degrees of the tooth surface accuracy. Therefore, by comparison in the tooth surface accuracy between the oil quenching and the gas quenching in the bar chart presented in the foregoing fashion, it is apparent that the bar of the gas quenching is smaller in value than the bar of the oil quenching, showing that the gas quenching is higher in the tooth surface accuracy than the oil quenching.
In conjunction with the oil quenching and the gas quenching having different characteristics as described above, a carburizing furnace capable of arbitrary selection of either one of the quenching processes is desired in recent years in order to meet all the needs regarding the production conditions for workpieces. Then, to realize such a carburizing furnace, various technologies have been proposed, including a technology in which the entire conveying path is vacuum-tightly sealed, and is disposed at a center of the furnace equipment, and a plurality of processing chambers provided as independent cells separate for each process step are disposed along the conveying path (see Japanese Patent Application Publication No. 6-137765 (JP-A-6-137765)), a technology in which a carriage that moves on the conveying path is provided with a vacuum-tightly sealed conveying chamber, and the conveying chamber is used for transfer of works (workpieces) between a plurality of processing chambers provided as cells (see Japanese Patent Application Publication No. 6-174377 (JP-A-6-174377)), etc.
An example of the cell-type carburizing furnace will be described. As an example of a reduced-pressure type carburizing furnace, more specifically, a cell-type reduced-pressure carburizing furnace 201 shown in FIG. 12A is constructed of a vacuum conveying chamber 202 disposed at a center, a plurality of cells 203, 204, . . . , 206 that are provided separately for each process step and that are arranged along the vacuum conveying chamber 202, etc. The cells 203, 204, . . . , 206 are each constructed as an independent cell structure, for example, heating cells 203, carburizing cells 204, . . . , a gas quenching cell 205, an oil quenching cell 206, etc. The oil quenching cell 206 is connected at a side thereof to the vacuum conveying chamber 202, and at another side to a conveyor 207 that conveys workpieces into and out of the furnace.
To perform the carburizing process on a workpiece, the workpiece conveyed by the conveyor 207 firstly passes through the oil quenching cell 206, and is conveyed to one of the heating cells 203 via the inside of the vacuum conveying chamber 202 (as shown by an arrow 1 in FIG. 12A). After being heated in the heating cell 203, the workpiece is conveyed to one of the carburizing cells 204 via the inside of the vacuum conveying chamber 202 (as shown by an arrow 2 in FIG. 12A). After being carburized in the carburizing cell 204, the workpiece is conveyed to the gas quenching cell 205 via the inside of the vacuum conveying chamber 202 (as shown by an arrow 3 in FIG. 12A). After being quenched in the gas quenching cell 205, the workpiece is conveyed via the inside of the vacuum conveying chamber 202, and passes through the oil quenching cell 206 again, and then is sent to the conveyor 207 (as shown by an arrow 4 in FIG. 12A). Incidentally, in the case where the oil quenching is performed after the carburizing process, the workpiece is oil-quenched when the workpiece is conveyed to the oil quenching cell 206 after being conveyed from one of the carburizing cells 204.
The use of the foregoing cell-type reduced-pressure carburizing furnace 201 makes it possible to arbitrarily select either one of the oil quenching and the gas quenching for use in the quenching process of a workpiece that is performed after a surface of the workpiece has been infiltrated with carbon (carburized) and the carbon in the surface has been diffused, so as to meet all the needs related to the production conditions for the workpiece. However, due to the layout of the furnace equipment, the cells 203, 204, . . . , 206 are rather sparsely located along the vacuum conveying chamber 202, so that a long moving time from one cell to another is required. Therefore, since the movement or conveyance from the carburizing cell 204 to the gas quenching cell 205 (or the oil quenching cell 206) requires a relatively long time, the temperature of the workpiece drops during the conveyance, so that the carburization hardening depth and the product accuracy vary greatly. Besides, in order to minimize the variations of the workpieces in the carburization hardening depth and the product accuracy, it becomes necessary to shorten the moving distance from one cell to another, which naturally limits the number of cells 203, 204, . . . , 206 that can be installed. As a result, the productivity of the cell-type reduced-pressure carburizing furnace 201 as a whole is rather low.
On the other hand, the vacuum conveying chamber 202 that extends connecting the cells 203, 204, . . . , 206 is large in size, and it is necessary to dispose a plurality of cell-type reduced-pressure carburizing furnaces 201, in order to secure a larger number of workpieces produced (the total number of workpieces that can be carburized by the cell-type reduced-pressure carburizing furnace 201 in a fixed amount of time). Therefore, a large installation space is needed, and the equipment-occupied area (i.e., the area of an installation space for one workpiece becomes large, so that the equipment cost increases.
Furthermore, in the vacuum conveying chamber 202, the flow lines (shown by the arrows 1 to 5 in FIG. 12A) representing the movements from one cell to another is complicated and intertangled, so that a complicated construction of the conveying mechanism results. Besides, since the inside of the cell-type reduced-pressure carburizing furnace 201 as a whole needs to be kept in a substantially vacuum state. Thus, the equipment as a whole needs to be constructed so as to have both good air tightness and good pressure resistance. Thus, the equipment cost increases.
There also exists a cell-type reduced-pressure carburizing furnace 301 as shown in FIG. 12B that is different from the foregoing carburizing furnace 201 despite of being of the reduced-pressure type as well. The cell-type reduced-pressure carburizing furnace 301 is constructed so that the processes from heating to cooling can be carried out in each of a plurality of independent cell chambers 302 and is constructed of a conveying path 303, and the plurality of cell chamber 302 disposed along the conveying direction of the conveying path 303. On the conveying path 303, a movable gas quenching chamber 305 having a conveying device 304 and a movable oil quenching chamber 306 having a conveying device 304 are provided independently of each other. In this construction, a workpiece is carburized as the workpiece is moved between the cell chambers 302 and the gas quenching chamber 305, or between the cell chambers 302 and the oil quenching chamber 306.
This cell-type reduced-pressure carburizing furnace 301 makes it possible to arbitrarily select either one of the oil quenching and the gas quenching for use in the quenching process of a workpiece that is performed after a surface of the workpiece has been infiltrated with carbon (carburized) and the carbon in the surface has been diffused, so as to meet all the needs related to the production conditions for workpieces. Besides, the gas quenching chamber 305 and the oil quenching chamber 306 provided independently of each other are each provided with a temperature keeping device, a vacuum pump, etc., so that, unlike the foregoing cell-type reduced-pressure carburizing furnace 201, temperature fall of a workpiece does not occur during the conveyance of workpieces. Therefore, there is no need to shorten the moving distance from one cell to another, so that the number of cell chambers 302 that can be installed will not be inconveniently limited.
However, the conveying devices 304, which each convey the gas quenching chamber 305 or the oil quenching chamber 306 independently from each other, have a long and large construction, and also have a complicated structure. Therefore, the equipment cost increases.
Besides, since the conveying devices 304 have a large conveying space, the installation space of the cell-type reduced-pressure carburizing furnace 31 is also large. Therefore, the equipment occupied area (i.e., the area of the installation space per workpiece) becomes large, so that the equipment cost increases.
Besides, in the case, for example, where a workpiece is moved between the gas quenching chamber 305 (or the oil quenching chamber 306) and the cell chambers 302, a substantially vacuum state needs to be maintained in each conveying device 304. An apparatus for creating such a vacuum state requires a complicated construction, which makes it difficult to secure reliability of the furnace equipment as a whole.
Furthermore, since the conveying devices 304, which each convey the gas quenching chamber 305 or the oil quenching chamber 306 independently of each other, have a long and large construction, the conveying speed of the conveying devices 304 is restrained to a low speed. Besides, since the cell chambers 302 are juxtaposed along the conveying path 303, the distance between two cell chambers 302 can be very long in some cases. In such a case, the moving time of the gas quenching chamber 305 or the oil quenching chamber 306 is long, so that a large amount of heat for keeping the temperature of workpieces is consumed in order to restrain variations of the product accuracy, resulting in increased running cost.